There has been considerable investigation into various forms of waveguide and slab CO.sub.2 lasers. (See, "The Waveguide Laser: A Review," Applied Physics, Vol. 11, pages 1-33 (1976)) A waveguide laser differs from a conventional laser in that the circulating light is guided over some portion of the propagation path and does not obey the laws of free space propagation. The term slab has been used to describe lasers having a rectangular discharge region defined between two planar surfaces. With respect to the subject invention, the term slab waveguide laser will be used to describe a laser having a rectangular discharge region defined between two narrowly spaced electrodes wherein light is guided in the narrow dimension between the electrodes yet is allowed to propagate in free space in the wider dimension.
The initial work in slab waveguide CO.sub.2 lasers was directed to flowing gas systems where the gas was excited with a DC discharge. (See, for example, "Optical-gain Measurements in a CW Transversedischarge, Transverse-gas-flow CO.sub.2 :N.sub.2 :He Planar-Waveguide Laser," McMullen et al., Journal of Applied Physics, Vol. 45, No. 11, Nov. 1974, pg. 5084) Efforts to extend the DC excitation approach to sealed CO.sub.2 lasers were not particularly successful.
The first satisfactory excitation scheme for a sealed CO.sub.2 waveguide laser is described in U.S. Pat. No. 4,169,251, issued Sept. 25, 1979 to Laakmann. The laser disclosed in this patent is transversely excited by a high frequency RF drive. The discharge region is defined between a pair of spaced apart, elongated electrodes. The Laakmann patent teaches how to select the proper RF excitation frequency based upon the spacing between the electrodes. Excitation with the proper RF frequency is necessary to maintain a stable discharge.
The electrodes of the laser disclosed in the Laakmann patent are spaced apart by a pair of elongated dielectric members. The combination of the electrodes and dielectric members can be used to define both square and rectangular discharge regions. In the embodiment illustrated in the patent, the spacing between the dielectric members is narrow enough so that light is guided in both dimensions, that is, between the electrodes and the dielectric members.
In a effort to increase the power which can be generated for a given length of electrodes, the teachings in the Laakmann patent have been applied to the development of CO.sub.2 slab waveguide lasers, wherein the light is guided only between the narrowly spaced electrodes and is allowed to propagate freely in the wider dimension. One of the earliest reports on this type of laser can be found in "Radio-frequency Excited Stripline CO and CO.sub.2 lasers," Gabai, Hertzberg and Yatsiv, Abstract presented at Conference on Lasers and Electro-optics, June 1984. In this laser, a pair of spaced apart water-cooled X-band waveguide electrodes were excited with an RF discharge at frequencies ranging from 25 to 50 MHz. The spacing between the electrodes was on the order of 4.5 mm, which was suitable for guiding light. The wider dimension of the electrodes was 2.5 cm which allowed the light to freely propagate. Cavity experiments were performed with plane mirrors and a stable resonator. Additional information can be found in a subsequent paper by Yatsiv, entitled "Conductively Cooled Capacitively Coupled RF Excited CO.sub.2 Lasers," given at the Gas Flow & Chemical Lasers Conference, 1986 and published by Springer, Proceedings, 6th Int. Sym, pages 252-257, 1987.
Another CO.sub.2 slab waveguide laser is described in U.S. Pat. No. 4,719,639, issued Jan. 12, 1988 to Tulip. Similar to the device described in the Gabai paper, the discharge region in the Tulip device is rectangular and configured to guide light between the electrodes while allowing the light to propagate in free space in the wider dimension. The Tulip patent also discloses that for a slab laser, it can be desirable to use an unstable resonator structure in the non-waveguide direction. The unstable resonator described in Tulip includes one concave mirror and one convex mirror and is known in the art as a positive branch unstable resonator.
Still another slab waveguide laser is described in "CO.sub.2 Large-area Discharge Laser Using an Unstable-waveguide waveguide Hybrid Resonator," Jackson et al., Applied Physics Letters, Vol. 54, No. 20, page 1950, May 1989. As in the laser described in the Tulip patent, the laser of this latter article was provided with a positive branch unstable resonator in the nonwaveguide direction.
Still another slab waveguide laser is disclosed in U.S. Pat. No. 4,939.,738, issued Jul. 3, 1990 to Opower. This slab waveguide laser is also provided with a positive branch unstable resonator.
Many of the initial CO.sub.2 slab waveguide laser designs reported in the prior literature while showing promise, have not been suitable for commercial exploitation. More particularly, most of the lasers described were essentially for experimental purposes and little effort was expended to overcome problems faced when attempting to operate the lasers at high power levels for extended periods of time. For example, in order to provide a commercially acceptable laser, various design issues must be addressed including mirror assemblies, cooling systems and electrode support structures.
Another problem with the prior art slab lasers is that their resonator structures included mirrors which were located quite close to the discharge and therefore subject to rapid degradation. This resonator construction followed the conventional wisdom of waveguide laser design which specifies the optimum placement and radius of the resonator mirrors. More specifically, prior art theory specified that the mirrors should either be located very close to the ends of the waveguide elements or spaced a large distance away. By placing the mirrors close to the end of the waveguide, it was felt that all of the light could be forced back into the waveguide channel. A cavity design where the mirrors are placed close to the end of the waveguide was said to have a Type I resonator. Mirrors used in a Type I resonator were typically flat, or had a very large radius of curvature. While a Type I resonator is fine for experimentation, degradation of the mirrors due to exposure to the nearby discharge renders this approach unsuitable for commercial applications.
The prior art waveguide theory also specified that there would be two additional locations, spaced far from the end of the waveguide, where mirrors could be placed and losses could still be minimized. These locations were a function of the separation between the electrodes and for convenience were labeled R and R/2. If the mirrors were placed at a distance R from the end of the waveguide, the resonator was called a Type II. If the mirrors were located at the distance R/2, the resonator was called as Type III. The radius of curvature of the mirrors in both a Type II and Type III resonators is equal to R. The distance R in a typical waveguide configuration turns out to range from about 10 cm to one meter. This additional spacing of both mirrors away from the ends of the waveguide is unacceptable in a commercial laser design since it would add additional space to the laser package and create potential alignment stability problems.
Accordingly, it is an object of the subject invention to provide a new and improved CO.sub.2 slab waveguide laser.
It is a further object of the subject invention to provide a CO.sub.2 slab waveguide laser which is stable and generates a high power output for a given length.
It is another object of the subject invention to provide a CO.sub.2 slab waveguide laser having an improved resonator structure.
It is still a further object of the subject invention to provide a CO.sub.2 slab waveguide laser having a negative branch unstable resonator in the nonwaveguide direction.
It is still another object of the subject invention to provide a CO.sub.2 slab waveguide laser having the resonator mirrors spaced away from the ends of the guide to reduce degradation.
It is still a further object of the subject invention to provide a CO.sub.2 slab waveguide laser having the resonator mirror spacing governed by the geometry of the negative branch unstable resonator.
It is still another object of the subject invention to provide a CO.sub.2 slab waveguide laser having an improved electrode support structure.
It is still a further object of the subject invention to provide a CO.sub.2 slab waveguide laser having an electrode support structure which allows for thermal expansion of the electrodes.
It is still another object of the subject invention to provide a CO.sub.2 slab waveguide laser having an electrode support structure which does not confine the discharge.
It is still a further object of the subject invention to provide a CO.sub.2 slab waveguide laser having an improved system for cooling the electrodes.
It is still another object of the subject invention to provide a CO.sub.2 slab waveguide laser having improved mirror mounts which allow adjustment from outside the sealed laser housing.
It is still a further object of the subject invention to provide a means for preionizing the discharge to facilitate operation.
It is still another object of the subject invention to provide a laser for generating short, high energy pulses suitable for treating tissue.